For many reasons, it is highly desirable to provide generally opaque thermoformed plastic containers with transparent window areas through which the packaged product can be viewed. As used herein, the term opaque refers to the external appearance of the material, which may actually be translucent, but which is not transparent to an optical image. In the case of conventional thermoformed containers made of extruded thermoplastic sheet material, the only way to provide such transparent windows is by coating the remainder of the plastic material with a layer of opaque paint or ink, e.g. by a silk screening operation. For added protection and to provide a very smooth and a glossy appearance, the opaque layer may, in turn, be overlaid with a coating of transparent material. This is obviously a relatively slow and expensive process but is nevertheless used for packaging products for which apparent packaging quality is an important sales consideration, e.g. cosmetics, jewelry, etc.
In recent years, foamed thermoplastic material has become increasingly important in the packaging industry because of its relatively low cost, its inherent opacity, its insulating qualities and its ability to protect packaged items from damage by absorbing impact. In many cases, foamed plastic container material is provided with an outer skin to make it still more attractive and resistant to being soiled, but such a skin is usually very thin and does not give the material the same appearance as extruded unfoamed pigmented or coated plastic. Because of its characteristic appearance and its widespread use with food products or other commodity type items, such packaging is not normally associated with items for which the package, sometimes referred to as a "jewel box" package, is intended to convey the impression of quality, prestige, distinctiveness, etc.